The present invention is directed to a multi-nozzle spray desuperheater. More particularly, the invention provides a new and improved spray tube assembly having a plurality of nozzle means whereby cooling water may be controllably injected into a fluid stream, typically steam, to selectively maintain the stream at a predetermined temperature level.
Generally, systems which depend upon steam in their operation are designed to operate with the stream at an optimum, predetermined temperature and pressure. In many instances, the steam being provided to the system is in a superheated condition and/or at a temperature considerably higher than is desirable for use in the utilization system. Typically, under such circumstances, a desuperheater is provided in the steam inlet line to cool and regulate the temperature of the incoming steam to the temperature level optimum for use in the system. Pursuant to a well known prior art proposal, the steam is cooled by the introduction of a spray of cooling water into the steam flow. In this manner, the water droplets of the cooling water spray mix with the steam flow thus removing heat from the steam and lowering its temperature. A common design for a prior art water spray type desuperheater provides for an adjustable valve with a discharge opening positioned in the steam line and arranged to inject a cooling water spray into the steam flow. Operation of the valve is regulated by a temperature sensor arranged to detect the temperature of the flowing stream at a point somewhat downstream from the position of the discharge opening of the adjustable valve. In many instances, the discharge opening of the valve is designed to inject the water in the form of a conical spray to insure a thorough mixing of the cooling water within and throughout the stream flow to thereby achieve a uniform temperature regulation.
In order for a water spray type desuperheater to be effective in an industrial scale steam utilization system, it is essential that the desuperheater device be operable to precisely control the amount of cooling water injected into the steam flow and to inject the cooling water in a spray form that will readily withdraw thermal energy from the steam. Moreover, the cooling water spray must follow a flow geometry whereby the cooling effects of the water flow are uniformly distributed throughout the steam flow. As should be understood, the inability of a water spray type desuperheater to precisely control the amount and characteristics of the water sprayed into the steam will greatly detract from the effectiveness of the desuperheater as a means to precisely control the temperature of the steam. A failure to precisely control the amount of water injected will directly result in an imprecisely determinable temperature change for the steam. Likewise, a non-uniformly dispersed water injection or a water injection lacking the necessary mist characteristics to faciliate a rapid evaporation of the water will result in a poorly controlled and uneven temperature reduction within and throughout the steam flow. In addition, the inability of the water injection to efficiently evaporate into the steam flow will cause an accumulation of water to form on the steam pipe in the vicinity of the discharge nozzle. This accumulation of water will eventually evaporate in a non-uniform and uncontrolled heat exchange between the water and the steam flowing around the water accumulation.
Another important consideration in the design of water spray-type desuperheaters is rangeability. This refers to the range of operation of the desuperheater (expressed in flow capacity utilization from maximum flow when fully open to a minimum flow) during which the desuperheater control over the water flow permits an effective cooling of the steam. Typically, a valve tends to lose flow control in the very low end of its range of operation, as when the valve plug is displaced a small distance from the valve seat. Consequently, adjustable valve type desuperheaters tend to have a rangeability which is less than desirable in many practical desuperheater applications.
One prior art proposal, disclosed in U.S. Pat. No. 4,130,611, addressed the optimum operating characteristics for a water spray type desuperheater, discussed above. Pursuant to the disclosure of U.S. Pat. No. 4,130,611, the desuperheater includes a multi-nozzle spray tube positioned in a steamline to inject a plurality of cooling water sprays into the steam flow. An axially movable valve plug is arranged within the spray tube and is utilized to progressively and sequentially expose the several nozzle means to water flow. The valve plug comprises a generally solid, unbalanced valve plug member tightly received in a leak-tight relation within the spray tube. The solid valve plug member may interrupt water flow to the desuperheater spray tube by mating with a valve seat arranged in the upper portions of the spray tube. Each of the nozzle means is in fluid communication with the water flow path portions of the spray tube and the several nozzle means are positioned about the cylindrical surface of the spray tube in a predetermined array. As the valve plug is lowered away from the valve seat and into the spray tube, the valve plug will sequentially and progressively expose the various nozzle means to fluid flow. Moreover, each of the nozzle means communicates with the water flow path portions of the spray tube through a plurality of small ports. Accordingly, the amount of water flowing into each of the nozzle means will gradually increase as the valve plug passes by the particular nozzle means and gradually uncovers the several port openings interconnecting the particular nozzle means with the water flow path portions of the spray tube. In this manner, the proposal of U.S. Pat. No. 4,130,611 provides a means to accurately control the total amount of water injected into the steam flow. In addition, each of the nozzle means is arranged and configured to impart a spiral motion to the water whereby the water injected by each of the nozzle means into the steam flow will be in the form of a swirling spray traveling along an expanding helical path to achieve a thorough and uniform mixing of the water and steam and to facilitate a controlled heat exchange between the cooling water and the superheated steam.
It is a primary objective of the present invention to provide a new and improved multi-nozzle, water spray type desuperheater including features affording a higher degree of controllability over the cooling water throughout a greater range of operation than has been possible with heretofore known desuperheater designs. Generally, the desuperheater of the present invention comprises a head assembly and a spray tube assembly. The head assembly is designed to mount the automatic control means for the desuperheater and includes integral mounting and cooling medium flanges to mount the spray tube assembly to a main steamline and to provide fluid communication between the spray tube assembly and a source of cooling water. The head assembly supports the spray tube assembly whereby the spray tube assembly extends into the steamline with the longitudinal axis of the spray tube assembly being perpendicular to the direction of flow of the steam and the nozzle means being located within the center of the steam flow. In this manner, the multiple nozzles of the spray tube assembly will be positioned to effectively inject cooling water into the steam flow, as will be fully discussed below.
Pursuant to an important feature of the invention, the spray tube assembly comprises a cage structure including an internal water flow path portion connected to the source of cooling water and a multiple nozzle plate subassembly mounted to the cage structure. A plurality of slot-like openings are formed in the cage structure wall to provide multiple fluid flow paths for flow communication between the internal water flow path portions of the cage structure and the several nozzle means of the multiple nozzle plate subassembly. A generally hollow cylindrical piston is arranged for controlled axial movement within the water flow path portions of the cage structure and is formed to be of a length sufficient to physically cover all of the slot-like openings when the piston is in its lowermost axial position. The piston is movable toward and away from a mating relation between the lower end edge of the piston and a valve seat formed at the lower end of the internal water flow path portions. The valve plug forming piston is formed to include several ports arranged at the top portions of the piston whereby the cooling water will flow from the source of water into and through the hollow valve plug toward the valve seat. The valve plug ports and resulting cooling water flow through the valve plug act to balance the valve plug whereby there are no water pressure effects acting upon the plug. This will assure highly controllable and accurate piston movement during the operation of the desuperheater.
Moreover, the piston is provided with sealing means so as to be in a generally leak-tight relation with the water flow path of the cage structure. This will prevent any water leakage between the exterior slot-covering portions of the piston and the internal wall portions of the water flow path. Significantly, the sealing means is arranged to permit some limited water flow around the outside of the piston in a volume defined by the sealing means and a clearance between the lowermost portions of the piston and the water flow path wall. When the piston is seated against the valve seat, the piston, valve seat and sealing means prevent any water flow from occurring and the piston physically covers all of the slot-like openings.
In accordance with the invention, the several nozzle means are arranged in a predetermined array across the surface of the multiple nozzle plate subassembly with each of the nozzle means being in flow communication with a complementary slot opening of the cage structure such that cooling water may flow from the water flow path portions of the cage structure into the particular nozzle means. The several nozzle means and their complementary slot-like openings are arranged and configured whereby the nozzle means will be progressively and sequentially exposed to water flow as the piston is lifted off the valve seat and gradually, controllably moved upwardly within the water flow path portions of the cage structure. After the piston is lifted off the valve seat, a portion of the cooling water will undergo a flow reversal as the water flows from the interior of the piston, around the lower end edge of the hollow piston and up into the volume defined by the sealing means and the above-described clearance. The water flow within the clearance is then throttled by the sealing means across the first slot-like opening exposed to fluid flow by the upwardly moving piston. The water flow reversal, sealing means throttling effect contributes to the controllability of the water flow by the desuperheater as it flows from the piston and into the slot-like opening.
An important feature of the invention includes a critical spacing between the sealing means and the lowermost valve seat-engaging end of the valve plug piston. More specifically, the sealing means is arranged to overlie the lowermost slot-like opening associated with the first-to-be-opened nozzle means when the piston is seated against the valve seat. The amount of spacing between the sealing means and the lowermost end of the piston is fixed such that the lowermost slot-like opening is substantially exposed (e.g., approximately 50% exposed) to fluid flow in the clearance immediately upon the lifting of the piston from the valve seat, e.g., within the first sixteenth of an inch of piston displacement. In this manner, the first nozzle means will begin to operate at a substantial percentage of its flow capacity, e.g., at approximately 50% of flow capacity as soon as the piston is lifted from the valve seat. Typically, at 50% capacity, the nozzle water injection will be in a fine mist spray form rather than in the relatively large water droplet dribble form typical of low flow rate nozzle operation. Accordingly, the desuperheater will be effective to inject an efficient cooling spray from the instant the piston is lifted from the valve seat and prevent any water accumulation due to a dribble of relatively large water droplets. What is important is that the initial operation of the first-to-operate nozzle means be at a flow capacity sufficient to provide immediate mist discharge.
To advantage, the water flow reversal, sealing means throttling effect will occur with respect to each of the remaining slot-like openings as the particular opening is being uncovered by the upwardly moving piston. The remaining slots will initially be fully covered by the piston and each of the remaining nozzle means will commence operation from zero flow capacity. However, as will be fully described below, the nozzle means are arranged to be sequentially exposed to fluid flow from the lowermost nozzle means to the uppermost nozzle means such that any water droplets emitted by a nozzle during the initial period of operation will fall into the spray of a lower nozzle and be atomized thereby. Inasmuch as the first nozzle commences operation with a spray, no water droplets will be able to accumulate on the steam pipe. In this manner, the amount and flow characteristics of the cooling water injected into the steam flow may be precisely controlled in accordance with the axial position of the balanced piston for substantially the full working stroke of the piston from a minimal displacement from the valve seat to full open operation. The piston may be continuously modulated through its work stroke to uncover or cover some or all of the nozzle means whereby the amount of cooling water injected into the steam flow may be accurately, continuously controlled to regulate the temperature of the steam. The present invention therefore provides precise flow control throughout a maximum range of operation.
Each of the nozzle means is provided with a swirl inducing structure to induce a high velocity swirling motion to the cooling water prior to discharge by the nozzle means into the steam flow. The swirling intensifies the mechanical breakdown action of the discharge nozzle and produces a swirling cone-shaped mist within the steam flow. The fine mist characteristics of the cooling water injected into the steam flow insures a rapid absorption of the cooling medium by the steam and optimum desuperheater efficiency in uniformly and controllably reducing the temperature of the superheated steam. As an additional feature of the invention, the multiple nozzle plate subassembly mounts the various nozzle means across a flat rectangular surface whereby the various sprays emitted by each of the nozzle means will undergo a swirling interaction with one another to provide a narrow, cone-shaped pattern. Such a pattern will tend to keep the water particle injection near the center of the steamline, where fluid turbulence is the greatest. The precise number, size and location of each of the nozzle means, as well as the volume of each of the complementary slot-like openings, may be designed to accommodate various cooling medium flow requirements as dictated by the particular desuperheater application. As a general principle, the invention contemplates that the various nozzle means are arranged relative to one another such that the progressive, sequential exposure of the nozzle means to fluid flow by the action of the upwardly moving piston results in a modified equal percentage characteristic. In other words, the nozzle means will be uncovered by the piston to achieve a gradual continuous increase in cooling water flow over the entire working stroke of the piston.
Empirical testing of prototype models of the invention indicate that the novel design features disclosed herein provide a desuperheater having a range of operation of precisely controlled cooling water flow significantly greater than has been obtainable with heretofore available water spray-type desuperheaters. Indeed, the prototype models have proven to be operable to achieve efficient cooling of a steam flow over substantially the full working stroke of the piston. Moreover, the mist characteristics of the injected water spray enable the spray to efficiently cool a steam flow traveling at a relatively slow flow rate. This is due to the fact that the precise flow control, mist geometry and water particle size within the injected mist achieved by the desuperheater of the present invention result in efficient cooling despite the minimum flow turbulence typical in low flow rate steam flows. Thus, the present invention provides a desuperheater with a significantly expanded range of operation and which is effectively operable in a wide range of practical applications.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawings.